There are two fundamentally different techniques to guide a missile to a target. The first approach, called the proportional navigation technique, is commonly used because of its simplicity and effectiveness in many applications. Most notably, the proportional navigation approach is used in complex intercept situations, where numerous system errors are present or the target is moving in an unpredictable way. The second approach, called the filter/guidance law approach has certain advantages over the proportional navigation approach for targets that are not thrusting and are undergoing no aerodynamic effects, such as a satellite. In such a case, the system errors required to accurately model the intercept are few in number since the target exhibits a known or predictable thrust profile. The filter/guidance law approach determines information about the intercept (such as target position and velocity) using a guidance filter and utilizes this information, in a way defined by a guidance law, to direct the missile to the target.
The function of a filter/guidance law approach used during the terminal intercept phase of flight is to direct an in-flight vehicle to an accurate intercept of a target. The terminal intercept phase begins at target acquisition and ends when the vehicle intercepts the target. The filter/guidance law approach can be subdivided into the performance of two distinct tasks: the accurate determination of the position and velocity of the target relative to the vehicle and the use of this information to direct the vehicle. The relative position and velocity of the target can be determined using a Kalman filter or some equivalent technique. This aspect of the filter/guidance law approach is addressed in applicant's patent entitled "A Method of Kalman Filtering For Estimating the Position and Velocity of a Tracked Object," U.S. Pat. No. 5,051,751, issued Sept. 24, 1991. The second task, the utilization of the relative position and velocity information to direct the vehicle to the target, is referred to hereinafter as a guidance law or method.
The effectiveness of any guidance law relies greatly on the generation of estimated time of impact information in order to schedule events. No guidance law is presently capable of using simple passive devices for generating an estimated time of impact. Currently, existing guidance laws obtain this information through the use of active laser or radar systems. However, use of these systems adds weight and complexity to the design of the missile. Furthermore, numerous guidance laws have been developed for guiding liquid fuel motor driven missiles. Unfortunately, liquid fuel is quite toxic and is therefore unsafe in many stowage, handling and operations applications. Solid fuel, on the other hand, is relatively safe. The primary disadvantage of solid fuel motors is that each burn imparts a thrust of a predetermined magnitude. Unlike a liquid fuel, once a solid fuel is ignited, the magnitude and duration of the burn is difficult to alter. This poses a significant constraint on the capability of the vehicle and the design of the guidance law.